Illuminated rotatable knobs are disclosed in DE 35 35 881 C2 which describes an illumination apparatus formed from several light conductors and in which the position of the rotatable knob is indicated by corresponding illumination of a scale. This apparatus was developed especially for installation in a motor vehicle. However, a loss of light is incurred by the twisting of the light conductors while using the rotatable knob. Further, the lighting arrangement is very complex in its design.
DE 197 12 294.9 of Preh-Werke GmbH & Co. KG (the assignee of the present invention), published on Oct. 1, 1998, discloses a rotatable knob of a rotary potentiometer comprising a light-transmitting body. A light entry surface is designed in the rotational center thereof, ahead of which a light source is placed. In this case, the light-transmitting body is bounded in its peripheral area with a transparent housing part. This transparent housing part carries a scale visible from the outside. A transparent, light-conducting annulus with a path of light such that a total reflection surface is created, by which the path of the light beams is diverted, is placed between the peripheral area of the light-transmitting body of the rotatable knob and the scale. Again, in this case, experience shows that only a weak illumination of the scale is achieved.